Constrain Transforms Empty to Sphere Vertex (Not Sphere Itself)
You can use "make vertex parent" to do that:



It is some kind of the invert of the hook:


Place the empty as wanted
Select it
Shift select the sphere
Enter edit mode for the sphere (maintaining the empty selected)
Select the wanted vertex of the sphere + 2 other vertices: because 3 vertices are needed to define an orientation plane
The make them parent of the empty using CtrlP 




The result is visible here: